Wire bonding has been used in integrated circuit (IC) packaging since the inception of IC technology. In wire bonding, an electrical connection is established between an IC and an IC package substrate by means of wire connections to the bond pads on the IC. Nevertheless, despite its long history and continual refinement, wire bonding is being replaced to some extent by more advanced packaging technologies. Among the common IC interconnection technologies replacing wire bonding is flip chip bonding. In contrast to wire bonding, flip chip bonding electrically connects an IC to an IC package substrate by means of conductive solder bumps attached to each of the bond pads on the IC. Packaging using flip chip bonding has been shown to offer advantages in size, performance, flexibility, reliability and cost over other packaging technologies, including wire bonding. Despite these advantages, however, wire bonding still accounts for about 90% of today's IC interconnects.
For many kinds of ICs, copper metallization is rapidly replacing aluminum metallization because of copper's low cost and high conductivity when compared to aluminum. This transition has created new issues for both wire bonding and flip chip bonding. Copper is electrochemically active and migrates in an electrical environment. Moreover, copper corrodes to form copper oxide, an insulator, when exposed to ambient moisture and forms undesirable alloys with common materials used in ICs. As a result, conventional wire bonding and flip chip bonding technologies are frequently incompatible with bond pads comprising bare copper.
In response to these issues, copper bond pads are frequently capped in a thin layer of aluminum before they are wire bonded or flip chip bonded. This use of aluminum capping layers is described in, for example, U.S. Pat. No. 5,785,236 entitled “Advanced Copper Interconnect System that is Compatible with Existing IC Wire Bonding Technology,” which is incorporated herein by reference. The aluminum capping layer protects the copper surface against corrosion and provides a bondable surface for wires or solder bumps. Even so, this additional aluminum capping layer comes at a significant cost in terms of performance and processing. In terms of performance, the aluminum capping layer degrades thermal conductivity compared to copper. This means that the rate of heat dissipation from the IC to the IC package is lessened, thereby resulting in either decreased IC performance and reliability or the need to upgrade to a more expensive package. With respect to processing, aluminum capping layers on copper are also susceptible to delamination, especially at the corners of the bond pads. Such delamination can have a catastrophic effect on IC manufacturing yield. Moreover, attempts to correct this corner delamination by modifying the shape of the bond pads from a square or rectangle into, for example, an octagon shape may further degrade thermal conductivity. Bond pads that are not square or rectangular will also tend to create issues with the pattern recognition systems on typical assembly and probe equipment.
For the aforementioned reasons, there is a need for apparatus and methods which improve the delamination resistance and thermal conductivity of bond pads, especially those comprising an aluminum capping layer on a copper metallization layer.